As is well known, prestressed concrete (PC) construction methods for introducing prestress are used in large cylindrical concrete structures such as ground tanks for storing liquefied natural gas (LNG) or liquefied petroleum gas (LPG). The PC construction methods include methods of using bonded PC steel members as PC steel members and methods of using unbonded PC steel members as PC steel members. The PC construction method using the bonded PC steel members has the following processes. First, sheaths are arranged before concrete is poured. Bonded PC steel members such as PC steel rods, PC steel wires, or PC steel stranded wires (PC steel strands), are inserted into the sheaths before or after the concrete is cured, and are strained when the concrete reaches the desired strength. Afterwards, a material such as cement milk enters the sheaths under pressure in order to perform an anti-corrosion treatment and to bind and integrate the bonded PC steel members and the concrete. On the other hand, in the PC construction method using the unbonded PC steel members, grease is applied to PC steel members, and their surroundings are covered with sheaths.
As a prestressed concrete roof for a cylindrical tank, as disclosed in, for instance, Patent Documents 1 and 2, a technique for arranging PC steel members at an outer circumferential side of a discoid roof in a radial direction and thereby supporting strength against bending moment generated adjacent to the outer circumference side of the roof is known.